The current spring reverbs in the FM9 are objectively lacking. But the new ones in the Axe-FX are
great. So that's a legitimate one.
Otherwise, there are a few things that seem like low-hanging fruit. A proper CE-1 model, for instance -- there's even a "roll-your-own" preset for it in the FM9, but why not have it as a dedicated effect type? (I'd also reaaaally love for it to be fully modeled with its crappy clippy pre-amp and hi/lo circuits, which each play a big role in its tone.)
Nitpicking aside, almost everything sounds so great that you can get what you need one way or another. I do think the real-world pedal complement of Helix is broader, though. (he says with a wink and a nudge... seems that's how you "inspire" new stuff in Fractal
)